X-ray scanner systems that are used to detect foreign objects and contaminants in food products and pharmaceutical products belong to the known state of the art. For safety reasons, the radiation in a scanner system or irradiation system of this type needs to be contained inside a cabinet-style enclosure with an entrance and an exit opening, and with a conveyor belt for transporting articles that are undergoing the inspection or irradiation through the entrance opening, through the inspection or irradiation area inside the enclosure, and out through the exit opening, as the articles pass through the inspection or irradiation system. In particular, the invention concerns the radiation-shielding curtains that are arranged at the entrance and exit openings of the cabinet-style enclosure of the system. A commonly familiar example of such an X-ray system with a conveyor belt and shielding curtains are the baggage inspection systems used at airports. The invention further includes radiographic inspection systems and irradiation systems equipped with at least one shielding curtain according to the invention.
In their most common form, the shielding curtains are vertically slit sheets of rubber or of a rubber-like material containing a radiation-blocking component such as lead oxide or tungsten, for example as a sandwiched laminate or in distributed form. An entire entrance- or exit curtain can consist of a single sheet, but typically two or three sheets are arranged one after another at both the entrance and the exit opening of the inspection cabinet. Most curtains are configured as a close coupled pair on one hanger with the slits of one curtain offset against the slits of the other, so as to minimize the leakage, as the individual strips can sometimes twist and be distorted.
With the large numbers of objects passing through an inspection system, the impact and friction between the objects and the curtain will cause wear and tear on the latter. In addition, if the objects being inspected are for example unpackaged meats, poultry or fish, the curtains (as well as all other exposed parts of the system) will be subjected to rigorous cleaning and sanitizing, typically with steam or hot water. As a result, the curtain may become brittle, and the unpackaged food products could become contaminated by fragments of the crumbling curtain material.
A state-of-the-art X-ray inspection system with radiation-shielding curtains in the form of slit flexible sheets is described and illustrated in EP 2 194 373 A1. To alleviate the problem of mechanical wear and tear, an X-ray shielding curtain with a tungsten-containing layer is proposed which includes one or more protective layers of a polyolefin resin laminated on the tungsten-containing layer. Test results are presented to demonstrate improved resistance to peeling and chipping with 6 and 9 million passages of test objects. However, the aspect of cleanability, i.e. resistance to aggressive high-temperature cleaning processes, is not addressed in this reference.
Another example of an X-ray inspection system with radiation-shielding curtains in the form of slit flexible sheets is described and illustrated in JP 4351901 B2. A modular curtain unit with three slit curtain sheets, a suspension frame and a quick-release mounting arrangement is proposed to facilitate cleaning, maintenance and replacement of the curtains. Like the first example, this system with its slit rubber curtains does not appear to be suitable for applications that involve the heavy-duty cleaning and sanitizing which is required in the food industry.
In an X-ray inspection system described in JP 4796333 B2, the objective is to reduce mechanical wear and tear on the radiation-shielding curtains, specifically due to impact and friction at hard corners and edges of block-shaped objects such as products in boxes. The objects have to be of identical size and shape and lined up in a defined travel path on the conveyor belt. The curtain has hard, metallic sections meeting the corners and edges of the objects, and soft flexible sections meeting the front of the object or hanging outside the hard, metallic sections. The system can be set up for different-sized objects by adjusting the positions of the hard, metallic sections. Like the first example above, this system only aims to reduce mechanical wear and tear, but does not address the problems of cleaning and sanitizing the curtains with hot water and steam.
The foregoing examples illustrate a drawback which, in the experience of the applicant, is common to all shielding curtains made of rubber or any other flexible materials, i.e. their inability to meet the requirements of the food industry, particularly in applications where the shielding curtain comes into contact with unpackaged food products.
A radiation-shielding curtain described in JP 2012159355 A is divided into narrow vertical elements that are suspended from a curtain rod, analogous to the strips of the slit sheet curtains described above, but with the difference that the elements are stainless steel sheet metal stampings with a channel profile over part of their lengths for stiffness. The bottom ends of the curtain strips are curved in the downstream direction of the inspection path to make them glide more easily over the moving inspection objects. A stop device prevents swing-back of the curtain elements in the upstream direction and also keeps the curtain elements in a slightly inclined position which further eases the sliding contact between the inspection objects and the curtain elements.
A further X-ray inspection system which is described in JP 03175410 U has a radiation-shielding curtain divided into vertical elements like in all of the previous examples, but is specifically adapted to unpackaged food items which are transported through the inspection system in an open rectangular bin or other box-like container whose walls are higher than the top of the contents. The curtain elements overlap each other in such a way that when a bin travels through the curtain, any element pushed up by the moving bin will also lift up the next element on the side towards the center of the transport path. Thus, as long as the outermost elements are pushed up by the sidewalls of the moving bin, they will also hold up all of the elements lying between them, so that all elements in the path of the bin are lifted together as one flat sheet and no curtain element comes into contact with the contents of the open bin. Stainless steel and tungsten are mentioned as possible materials for the curtain elements, as well as the possibility of attaching a counterweight to each element above its point of suspension, which would allow the curtain to be pushed up more easily by the moving bin.
Of the five examples of state-of-the-art shielding curtains cited above, the first three, EP 2 194 373 A1, JP 4351901 B2 and JP 4796333 B2, are made at least in part of rubber or a rubber-like material which makes them unsuitable for applications where they would come into contact with unpackaged food, in particular meat, poultry and seafood, and would be subject to daily cleaning with hot water or steam.
In the fourth example, JP 2012159355 A, the curtain is made of stainless steel and at least in principle meets the requirement of cleanability, although there could be some concern about contaminants accumulating along the inside edges of the channeled profiles of the curtain elements. Furthermore, the sheet-metal curtain elements may not drape themselves closely against the objects moving through the curtain and there could be some scraping and jamming between the exposed lateral edges of the sheet-metal stampings and the objects moving through the curtain.
The radiation-shielding curtain of the fifth example, JP 03175410 U, if made of stainless steel and/or tungsten, could likewise meet the requirement of cleanability. On the other hand, this curtain with its overlapping elements is designed for the specific, limited application described above, which could actually prevent the curtain elements from gliding closely over the surface of irregular-shaped inspection objects. Also, if the curtain elements are made in the form of sheet-metal stampings, there would be the same concern about scraping and jamming between the exposed lateral edges of the curtain elements and the objects moving through the curtain.